Polyimides are excellent in heat resistance, electric insulation performance, mechanical properties, chemical resistance and the like, so that they are widely used in electric and electronic parts, airplane parts, car parts, peripheral circuits of semiconductors and the like, with the compaction and precision of these.
Since conventional polyimides represented by Kapton are insoluble in solvents, the methods are widely employed, in which aromatic tetracarboxylic dianhydrides and aromatic diamines are polycondensed in polar solvents at a temperature not higher than 10° C. to obtain polyamic acids that are precursors of polyimides, the obtained polyamic acids are applied to substrates and then heated at 250° C. to 350° C. to imidize the polyamic acids to polyimides.
Flexible printed boards made of a polyimide are subjected to a number of steps accompanying heat treatments at high temperatures, so that dimensional change occurs, which is problematic to processing of the boards. Thus, a material for forming a composite board for use as a flexible printed board or a film for TAB, which is prepared by coating copper foil with a polyimide to form a multi-layered board, and which is free from warping, is demanded.
To obtain a composite material free from warping or curling, it has been tried to make the coefficients of linear expansion of the copper foil and of the film material close.
As an example, it has been found that a flexible printed board with good adhesion and reduced warping may be obtained by employing a solvent-soluble polyimide and treating the polyimide film-coated substrate at a low temperature (Japanese Laid-open Patent Application (Kokai) No. 10-224017). A method in which a polyimide having ether imide group is employed (Japanese Laid-open Patent Application (Kokai) No. 8-217877), and a method in which a polyimide composition in which the polyimide components are variously changed (Japanese Laid-open Patent Application (Kokai) Nos. 741556 and 60-243120) have been tried.
On the other hand, polyimides are also applied to circuit boards or the like as photosensitive resins. They are used as negative-type photoresists by converting the polyamic acid to acrylic acid ester thereof, irradiating the resultant with light after adding a radical generator, thereby curing the irradiated portions, and dissolving the non-irradiated portions in an alkaline developer to form negative-type images. Since positive-type photoresists are more sensitive and give higher resolutions than the negative-type photoresists, a positive-type photoresist is demanded for forming patterns of through holes.
Novolak resins have phenolic hydroxyl groups, and upon being irradiated with light together with a photoacid generator, they come to be soluble in alkaline developers, so that they are widely used as positive-type photoresists. Polyimides having phenolic hydroxyl groups have been prepared, and it has been confirmed that upon irradiating such a polyimide together with a photoacid generator, they come to be soluble in alkaline developers so as to form positive-type images (Japanese Laid-open Patent Application (Kokai) No. 64-60630, U.S. Pat. No. 4,927,736, Japanese Laid-open Patent Application (Kokai) Nos. 3-209478 and 4-110348). It has been shown that polyimide having no phenolic hydroxyl groups do not form positive-type images (Japanese Laid-open Patent Application (Kokai) No. 3-209478).
Even the polyimides having no phenolic hydroxyl groups, by irradiating the polyimides with light together with a photoacid generator and by developing the resultant with an amino alcohol-containing developer, imide bonds are cleaved so that high molecular polyimides are made into low molecular polyimides and dissolved in the developer, so as to form positive-type images (PCT/JP98/04577).